Stationary source for marine electromagnetic surveying

Abstract

Disclosed are methods and systems for using electromagnetic sources that are substantially stationary in a body of water while electromagnetic field signals are detected with electromagnetic sensors towed through the body of water. An embodiment discloses an apparatus comprising: a vessel; a first electromagnetic source coupled to the vessel and configured to have a dipole moment oriented in a first direction; and a second electromagnetic source coupled to the vessel and configured to have a dipole moment oriented in a second direction, wherein the second direction is generally orthogonal to the first direction. Another embodiment discloses a system comprising: a sensor streamer configured for towing in a body of water, wherein the sensor streamer comprises a plurality of electromagnetic sensors disposed at spaced apart positions; and an electromagnetic source assembly configured for deployment at a substantially stationary position in the body of water.

What is claimed is: 
     
       1. An apparatus for geophysical surveying comprising:
 a vessel, wherein the vessel is a container, boat, or ship, wherein the vessel is configured for deployment at a substantially stationary position in a body of water, wherein the vessel is secured with an anchor to a bottom of the body of water; 
 a first electromagnetic source coupled to the vessel and configured to have a dipole moment oriented in a first direction; and 
 a second electromagnetic source coupled to the vessel and configured to have a dipole moment oriented in a second direction, wherein the second direction is generally orthogonal to the first direction. 
 
     
     
       2. The apparatus of  claim 1  wherein the first electromagnetic source and the second electromagnetic source are each coupled to the vessel by separate source cables. 
     
     
       3. The apparatus of  claim 2  wherein the vessel is a floating container configured for positioning at a substantially stationary position in a body of water. 
     
     
       4. The apparatus of  claim 1  wherein the first electromagnetic source comprises a pair of electrodes configured for spacing along a source cable in the first direction, and wherein the second electromagnetic source comprises a pair of electrodes configured for spacing along a source cable in the second direction. 
     
     
       5. The apparatus of  claim 4  wherein the pair of electrodes of the first electromagnetic source are configured for placement at different depths in the body of water, and wherein the pair of electrodes of the second electromagnetic source are configured for placement along or near a bottom of the body of water. 
     
     
       6. The apparatus of  claim 1  further comprising electromagnetic sensors coupled to the vessel. 
     
     
       7. The apparatus of  claim 1  wherein the apparatus further comprises a magnetic loop source secured to the vessel. 
     
     
       8. The apparatus of  claim 4  wherein the pair of electrodes of the first electromagnetic source are configured for placement at different depths in the body of water, and wherein the pair of electrodes of the second electromagnetic source are configured for placement along or near a bottom of the body of water, and wherein the pair of electrodes of the third electromagnetic source are configured for placement along or near a bottom of the body of water. 
     
     
       9. An apparatus for geophysical surveying comprising:
 a vessel, wherein the vessel is a container, boat, or ship, wherein the vessel is configured for deployment at a substantially stationary position in a body of water; 
 a first electromagnetic source coupled to the vessel and configured to have a dipole moment oriented in a first direction; and 
 a second electromagnetic source coupled to the vessel and configured to have a dipole moment oriented in a second direction, wherein the second direction is generally orthogonal to the first direction 
 wherein the first electromagnetic source comprises a pair of electrodes configured for spacing along a source cable in the first direction, and wherein the second electromagnetic source comprises a pair of electrodes configured for spacing along a source cable in the second direction 
 wherein the apparatus further comprises a third electromagnetic source comprising a pair of electrodes configured for spacing along a source cable in a third direction, wherein the third direction is generally orthogonal to the first and second directions. 
 
     
     
       10. The apparatus of  claim 9  wherein the first electromagnetic source, the second electromagnetic source, and the third electromagnetic source are each coupled to the vessel by separate source cables. 
     
     
       11. The apparatus of  claim 9  wherein the vessel is the container configured for positioning at the substantially stationary position in the body of water with an anchor secured to a bottom of the body of water. 
     
     
       12. The apparatus of  claim 9  further comprising electromagnetic sensors coupled to the vessel. 
     
     
       13. The apparatus of  claim 9  wherein the apparatus further comprises a magnetic loop source secured to the vessel. 
     
     
       14. A system for geophysical surveying comprising:
 a sensor streamer configured for towing in a body of water, wherein the sensor streamer comprises a plurality of electromagnetic sensors disposed at spaced apart positions, wherein the electromagnetic sensors are configured to detect changes in an electric field; and 
 an electromagnetic source assembly configured for deployment at a substantially stationary position in the body of water. 
 
     
     
       15. The system of  claim 14  further comprising a survey vessel configured for towing the sensor streamer in the body of water. 
     
     
       16. The system of  claim 15  wherein the electromagnetic source assembly comprises:
 a vessel configured for placement at a substantially stationary position in the body of water; 
 a first electromagnetic source comprising a pair of electrodes longitudinally spaced on a first source cable in a first direction, wherein the first source cable is secured to the vessel; and 
 a second electromagnetic source comprising a pair of electrodes longitudinally spaced on a second source cable in a second direction, wherein the second direction is generally orthogonal to the first direction, wherein the second source cable is secured to the vessel. 
 
     
     
       17. The system of  claim 16  wherein the electrodes of the first electromagnetic source are configured for placement at different depths in the body of water. 
     
     
       18. The system of  claim 17  wherein the electrodes of the second electromagnetic source are configured for placement along or near a bottom of the body of water. 
     
     
       19. The system of  claim 16  wherein the electromagnetic source assembly further comprises a third electromagnetic source comprising a pair of electrodes longitudinally spaced on a third source cable in a direction generally orthogonal to the first and second directions, wherein the third source cable is secured to the vessel. 
     
     
       20. The system of  claim 16  wherein the electromagnetic source assembly further comprises a magnetic loop source secured to the vessel. 
     
     
       21. The method of  claim 16  wherein the vessel configured for placement at the substantially stationary position in the body of water is a container, a ship or a boat. 
     
     
       22. A method of electromagnetic surveying comprising:
 actuating one or more electromagnetic sources while substantially stationary in a body of water; and 
 detecting electromagnetic field signals at electromagnetic sensors towed through the body of water. 
 
     
     
       23. The method of  claim 22  further comprising positioning a first pair of source electrodes in a substantially stationary position in the body of water such that the first pair of the source electrodes are longitudinally spaced on a first source cable in a first direction, and positioning a second pair of source electrodes in a substantially stationary position in the body of water such that the second pair of the source electrodes are longitudinally spaced on a second source cable in a second direction, wherein the second direction is generally orthogonal to the first direction. 
     
     
       24. The method of  claim 22  further comprising identifying a target area for an electromagnetic survey wherein the target area is positioned underneath a screening body, and towing the electromagnetic sensors on one side of the screening body while the one or more electromagnetic sources are positioned on an opposite side of the screening body. 
     
     
       25. The method of  claim 22  further comprising towing the electromagnetic sensors over an environmental zone in which electromagnetic source discharges are restricted while the electromagnetic source is positioned outside the environmental zone. 
     
     
       26. The method of  claim 22  wherein the one or more electromagnetic sources are secured to a vessel that is substantially stationary in the body of water, wherein the vessel is a container, a ship or a boat anchored to a water bottom. 
     
     
       27. The method of  claim 22  further comprising using a steering and/or propulsion system to maintain a vessel coupled to the one or more electromagnetic sources in a substantially stationary position. 
     
     
       28. The method of  claim 22  further comprising repositioning the one or electromagnetic sources in the body of water, and then repeating the steps of actuating the one or more electromagnetic sources and detecting electromagnetic field signals. 
     
     
       29. The method of  claim 24  further comprising moving the one or more electromagnetic sources to the side of the screening body on which the electromagnetic sensors were towed and then towing the electromagnetic sensors on the opposite side of the screening body. 
     
     
       30. An apparatus for geophysical surveying comprising:
 a first electromagnetic source configured for deployment at a substantially stationary position in a body of water and configured to have a dipole moment oriented in a first direction; 
 a second electromagnetic source configured for deployment at a substantially stationary position in the body of water and configured to have a dipole moment oriented in a second direction, wherein the second direction is generally orthogonal to the first direction; and 
 a third electromagnetic source configured for deployment at a substantially stationary position in the body of water and configured to have a dipole moment oriented in a third direction, wherein the third direction is generally orthogonal to the first direction and the second direction. 
 
     
     
       31. The apparatus of  claim 30  wherein the first electromagnetic source comprises a pair of electromagnetic source electrodes spaced on a first source cable, wherein the second electromagnetic source comprises a pair of electromagnetic source electrodes spaced on a second source cable, and wherein the third electromagnetic source comprises a pair of electromagnetic source electrodes spaced on a third source cable. 
     
     
       32. The apparatus of  claim 31  wherein the first source cable, the second source cable, and the third source cable are coupled to a vessel. 
     
     
       33. The apparatus of  claim 32  wherein the vessel is a container, a ship or a boat. 
     
     
       34. The apparatus of  claim 31  further comprising electromagnetic sensors coupled to the vessel. 
     
     
       35. The apparatus of  claim 31  wherein the apparatus further comprises a magnetic loop source secured to the vessel. 
     
     
       36. The apparatus of  claim 31  wherein the pair of electrodes of the first electromagnetic source are configured for placement at different depths in a body of water, and wherein the pair of electrodes of the second electromagnetic source are configured for placement along or near a bottom of the body of water, and wherein the pair of electrodes of the third electromagnetic source are configured for placement along or near the bottom of the body of water.